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Purpose F13640 (a.k.a. befiradol, NLX-112) is a highly selective 5-HT1A receptor ligand that was selected as a PET radiopharmaceutical-candidate based on animal studies. Due to its high efficacy agonist properties, [18F]F13640 binds preferentially to functional 5-HT1A receptors, which are coupled to intracellular G-proteins. Here, we characterize brain labeling of 5-HT1A receptors by [18F]F13640 in humans and describe a simplified model for its quantification. Methods PET/CT and PET-MRI scans were conducted in a total of 13 healthy male volunteers (29 ± 9 years old), with arterial input functions (AIF) (n = 9) and test–retest protocol (n = 8). Several kinetic models were compared (one tissue compartment model, two-tissue compartment model, and Logan); two models with reference region were also evaluated: simplified reference tissue model (SRTM) and the logan reference model (LREF). Results [18F]F13640 showed high uptake values in raphe nuclei and cortical regions. SRTM and LREF models showed a very high correlation with kinetic models using AIF. As concerns test–retest parameters and the prolonged binding kinetics of [18F]F13640, better reproducibility, and reliability were found with the LREF method. Cerebellum white matter and frontal lobe white matter stand out as suitable reference regions. Conclusion The favorable brain labeling and kinetic profile of [18F]F13640, its high receptor specificity and its high efficacy agonist properties open new perspectives for studying functionally active 5-HT1A receptors, unlike previous radiopharmaceuticals that act as antagonists. [18F]F13640’s kinetic properties allow injection outside of the PET scanner with delayed acquisitions, facilitating the design of innovative longitudinal protocols in neurology and psychiatry. Trial Registration. Trial Registration EudraCT 2017–002,722-21.

Alcohol-use-disorders are chronic relapsing illnesses, often co-morbid with anxiety. We have previously shown using the “drinking-in-the-dark” model in mice that the stimulation of the serotonin receptor 1A (5-HT1A) reduces ethanol binge-drinking behaviour and withdrawal-induced anxiety. The 5-HT1A receptor is located either on Raphe neurons as autoreceptors, or on target neurons as heteroreceptors. By combining a pharmacological approach with biased agonists targeting the 5-HT1A auto- or heteroreceptor and a chemogenetic approach (DREADDs), here we identified that ethanol-binge drinking behaviour is dependent on 5-HT1A autoreceptors and 5-HT neuronal function, with a transition from DRN-dependent regulation of short-term (6 weeks) ethanol intake, to MRN-dependent regulation after longer ethanol exposure (12 weeks). We further identified a serotonergic microcircuit (5-HTMRN→DG) originating from the MRN and projecting to the dentate gyrus (DG) of the hippocampus, that is specifically affected by, and modulates long-term ethanol consumption. The present study indicates that targeting Raphe nuclei 5-HT1A autoreceptors with agonists might represent an innovative pharmacotherapeutic strategy to combat alcohol abuse.

International audience; Background: The gold-standard treatment for Parkinson’s disease is L-DOPA, which in the long term often leads to levodopa-induced dyskinesia. Serotonergic neurons are partially responsible for this, by converting L-DOPA into dopamine leading to its uncontrolled release as a “false neurotransmitter”. The stimulation of 5-HT1A receptors can reduce involuntary movements but this mechanism is poorly understood. Objective: This study aimed to investigate the functionality of 5-HT1A receptors using positron emission tomography in hemiparkinsonian rats with or without dyskinesia induced by 3-weeks daily treatment with L-DOPA. Imaging sessions were performed “off” L-DOPA. Methods: Each rat underwent a positron emission tomography scan with [18F]F13640, a 5-HT1AR agonist which labels receptors in a high affinity state for agonists, or with [18F]MPPF, a 5-HT1AR antagonist which labels all the receptors. Results: There were decreases of [18F]MPPF binding in hemiparkinsonian rats in cortical areas. In dyskinetic animals, changes were slighter but also found in other regions. In hemiparkinsonian rats, [18F]F13640 uptake was decreased bilaterally in the globus pallidus and thalamus. On the non-lesioned side, binding was increased in the insula, the hippocampus and the amygdala. In dyskinetic animals, [18F]F13640 binding was strongly increased in cortical and limbic areas, especially in the non-lesioned side. Conclusion: These data suggest that agonist and antagonist 5-HT1A receptor-binding sites are differently modified in Parkinson’s disease and levodopa-induced dyskinesia. In particular, these observations suggest a substantial involvement of the functional state of 5-HT1AR in levodopa-induced dyskinesia and emphasize the need to characterize this state using agonist radiotracers in physiological and pathological conditions.

A high proportion of depressed patients fail to respond to antidepressant drug treatment. Treatment-resistant depression (TRD) is a major challenge for the psychopharmacology of mood disorders. Only in the past decade have novel treatments, including deep brain stimulation (DBS) and ketamine, been discovered that provide rapid and sometimes prolonged relief to a high proportion of TRD sufferers. In this review, we consider the current status of TRD from four perspectives: the challenge of developing an appropriate regulatory framework for novel rapidly acting antidepressants; the efficacy of non-pharmacological somatic therapies; the development of an animal model of TRD and its use to understand the neural basis of antidepressant non-response; and the potential for rapid antidepressant action from targets (such as 5-HT1A receptors) beyond the glutamate receptor.

Rationale The prevalence of depression is ever-increasing throughout the population. However, available treatments are ineffective in around one-third of patients and there is a need for more effective and safer drugs. Objectives The antidepressant-like and procognitive effects of the “biased agonists” F15599 (also known as NLX-101) which preferentially targets postsynaptic 5-HT1A receptors and F13714, which targets 5-HT1A autoreceptors, were investigated in mice. Methods Antidepressant-like properties of the compounds and their effect on cognitive functions were assessed using the forced swim test (FST) and the novel object recognition (NOR), respectively. Next, we induced a depressive-like state by an unpredictable chronic mild stress (UCMS) procedure to test the compounds’ activity in the depression model, followed by measures of sucrose preference, FST, and locomotor activity. Levels of phosphorylated cyclic AMP response element-binding protein (p-CREB) and phosphorylated extracellular signal-regulated kinase (p-ERK1/2) were also determined. Results F15599 reduced immobility time in the FST over a wider dose-range (2 to 16 mg/kg po) than F13714 (2 and 4 mg/kg po), suggesting accentuated antidepressant-like properties in mice. F15599 did not disrupt long-term memory consolidation in the NOR at any dose tested, while F13714 impaired memory formation, notably at higher doses (4–16 mg/kg). In UCMS mice, a single administration of F15599 and F13714 was sufficient to robustly normalize depressive-like behavior in the FST but did not rescue disrupted sucrose preference. Both F15599 and F13714 rescued cortical and hippocampal deficits in p-ERK1/2 levels of UCMS mice but did not influence the p-CREB levels. Conclusions Our studies showed that 5-HT1A receptor biased agonists such as F13714 and especially F15599, due to its less pronounced side effects, might have potential as fast-acting antidepressants.

The serotonin 5-HT1A receptor has attracted wide attention as a target for treatment of psychiatric disorders. Although this receptor is important in the pharmacological mechanisms of action of new-generation antipsychotics, its characterization remains incomplete. Studies based on in vitro molecular imaging on brain tissue by autoradiography, and more recently in vivo PET imaging, have not yielded clear results, in particular due to the limitations of current 5-HT1A radiotracers, which lack specificity and/or bind to all 5-HT1A receptors, regardless of their functional status. The new concept of PET neuroimaging of functionally active G-protein-coupled receptors makes it possible to revisit PET brain exploration by enabling new research paradigms. For the 5-HT1A receptor it is now possible to use [18F]-F13640, a 5-HT1A receptor radioligand with high efficacy agonist properties, to specifically visualize and quantify functionally active receptors, and to relate this information to subjects’ pathophysiological or pharmacological state. We therefore propose imaging protocols to follow changes in the pattern of functional 5-HT1A receptors in relation to mood deficits or cognitive processes. This could allow improved discrimination of different schizophrenia phenotypes and greater understanding of the basis of therapeutic responses to antipsychotic drugs. Finally, as well as targeting functionally active receptors to gain insights into the role of 5-HT1A receptors, the concept can also be extended to the study of other receptors involved in the pathophysiology or therapy of psychiatric disorders.

Pattern separation is a hippocampal process in which highly similar stimuli are recognized as separate representations, and deficits could lead to memory impairments in neuropsychiatric disorders such as schizophrenia. The 5-HT1A receptor (5-HT1AR) is believed to be involved in these hippocampal pattern separation processes. However, in the dorsal raphe nucleus (DRN), the 5-HT1AR is expressed as a somatodendritic autoreceptor, negatively regulates serotonergic signaling, and could thereby counteract the effects of hippocampal postsynaptic 5-HT1A receptors. Therefore, this study aims to identify how pre- and post-synaptic 5-HT1AR activity affects pattern separation. Object pattern separation (OPS) performance was measured in male Wistar rats after both acute and chronic treatment (i.p.) with 5-HT1AR biased agonists F13714 (0.0025 mg/kg acutely, 0.02 mg/kg/day chronically) or NLX-101 (0.08 mg/kg acutely, 0.32 mg/kg/day chronically), which preferentially activate autoreceptors or postsynaptic receptors respectively, for 14 days. Body temperature - a functional correlate of hypothalamic 5-HT1AR stimulation - was measured daily. Additionally, 5-HT1AR density (DRN) and plasticity markers (hippocampus) were assessed. Acute treatment with F13714 impaired OPS performance, whereas chronic treatment normalized this, and a drop in body temperature was found from day 4 onwards. NLX-101 enhanced OPS performance acutely and chronically, and caused an acute drop in body temperature. Chronic NLX-101 treatment increased doublecortin positive neurons in the dorsal hippocampus, while chronic treatment with F13714 resulted in a downregulation of 5-HT1A autoreceptors, which likely reversed the acute impairment in OPS performance. Chronic treatment with NLX-101 appears to have therapeutic potential to improve brain plasticity and OPS performance.

Novel 1-(1-benzoylpiperidin-4-yl)methanamine derivatives with high affinity and selectivity for serotonin 5-HT1A receptors were obtained and tested in four functional assays: ERK1/2 phosphorylation, adenylyl cyclase inhibition, calcium mobilization, and β-arrestin recruitment. Compounds 44 and 56 (2-methylaminophenoxyethyl and 2-(1H-indol-4-yloxy)ethyl derivatives, respectively) were selected as biased agonists with highly differential "signaling fingerprints" that translated into distinct in vivo profiles. In vitro, 44 showed biased agonism for ERK1/2 phosphorylation and, in vivo, it preferentially exerted an antidepressant-like effect in the Porsolt forced swimming test in rats. In contrast, compound 56 exhibited a first-in-class profile: it preferentially and potently activated β-arrestin recruitment in vitro and potently elicited lower lip retraction in vivo, a component of "serotonergic syndrome". Both compounds showed promising developability properties. The presented 5-HT1A receptor-biased agonists, preferentially targeting various signaling pathways, have the potential to become drug candidates for distinct central nervous system pathologies and possessing accentuated therapeutic activity and reduced side effects.

Tetrabenazine, a preferential inhibitor of the vesicular monoamine transporter type 2, depletes the brain monoamines dopamine, serotonin and norepinephrine. Tetrabenazine and deutetrabenazine (Austedo ®) are used to treat chorea associated with Huntington's disease. However, both compounds are known to aggravate Parkinsonism and depression observed in Huntington's disease patients. NLX-112 (a.k.a. befiradol/F13640) is a highly selective, potent and efficacious serotonin 5-HT 1A agonist. In animal models, it has robust efficacy in combating other iatrogenic motor disorders such as L-DOPA-induced dyskinesia and has marked antidepressant-like activity in rodent tests. In the present study, we investigated, in rats, the efficacy of NLX-112 to counteract tetrabenazine-induced catalepsy (a model of Parkinsonism) and tetrabenazine-induced potentiation of immobility in the forced swim test (FST, a model to detect antidepressant-like activity). The prototypical 5-HT 1A agonist, (±)8-OH-DPAT, and the 5-HT 1A partial agonist/dopamine D2 receptor blocker, buspirone, were used as comparators. Both NLX-112 and (±)8-OH-DPAT (0.16-2.5 mg/kg p.o. or s.c., respectively) abolished catalepsy induced by tetrabenazine (2 mg/kg i.p.). In comparison, buspirone (0.63-5.0 mg/kg p.o.) was ineffective and even tended to potentiate tetrabenazine-induced catalepsy at 0.63 mg/kg. In the FST, NLX-112 and (±)8-OH-DPAT (0.63 mg/kg) strongly reduced immobility when administered alone but also significantly opposed potentiation of immobility induced by tetrabenazine (1.5 mg/kg i.p.). Buspirone (0.63 and 2.5 mg/kg p.o.) had no effect by itself or against tetrabenazine. These results strongly suggest that selective and highly efficacious 5-HT 1A agonists, such as NLX-112, may be useful in combating tetrabenazine-induced Parkinsonism and/or depression in Huntington's disease patients.

NLX-101 and F13714 are selective, full efficacy, biased agonists of the 5-HT(1A) receptor. NLX-101 preferentially activates cortical post-synaptic 5-HT(1A) receptors whereas F13714 preferentially activates Raphe nuclei pre-synaptic 5-HT(1A) receptors. We compared NLX-101 and F13714 for their efficacy and potency to substitute for the discriminative cue produced by the prototypical, non-biased 5-HT(1A) receptor agonist, 8-OH-DPAT (racemate). Male and female Sprague-Dawley rats were trained to discriminate 8-OH-DPAT (0.1 mg/kg i.p., 20 min pre-treatment) from saline using a classical two-lever drug-discrimination procedure. 8-OH-DPAT (0.01 and 0.05 mg/kg i.p.) dose-dependently substituted for the training dose, with about 50 % responding on the 8-OH-DPAT-associated lever at 0.05 mg/kg. F13714 fully and very potently substituted for the training dose of 8-OH-DPAT from 0.018 mg/kg i.p., while NLX-101 only achieved full substitution at 0.5 mg/kg i.p., a dose which is known to also activate pre-synaptic 5-HT(1A) receptors. The 5-HT(1A) receptor partial agonist, buspirone, partially substituted (~80%) at 1 and 2 mg/kg i.p., doses which also decreased response rates. F13714 decreased response rates at 0.05 mg/kg. The selective 5-HT(1A) receptor antagonist WAY-100,635 (1 mg/kg s.c., 40 min pre-treatment) elicited almost no responding on the 8-OH-DPAT-associated lever by itself, but blocked the discriminative stimulus effects produced by administration (20 min pretreatment) of 8-OH-DPAT (0.1 mg/kg), F13714 (0.025 mg/kg), NLX-101 (0.5 mg/kg), or buspirone (1 mg/kg). These data suggest that the discriminative cue produced by 0.1 mg/kg i.p. 8-OH-DPAT results from activation of pre-synaptic 5-HT(1A) receptors. They also further demonstrate the distinct profiles in behavioral models of 5-HT(1A) receptor biased agonists.

The rise in obesity prevalence has been linked to overconsumption of high-sugar containing food and beverages. Recent evidence suggests that chronic sucrose consumption leads to changes in serotonergic neuroplasticity within the neural circuits involved in feeding control. Although there is a relationship between serotonin signalling in the brain and diet-induced obesity, the specific serotonin (5-HT) receptors or pathways involved remain unknown. The 5-HT

Cerebral ischemia due to stroke or cardiac arrest greatly affects daily functioning and the quality of life of patients and has a high socioeconomic impact due to the surge in their prevalence. Advances in the identification of an effective pharmacotherapy to promote neuroprotection and recovery after a cerebral ischemic insult are, however, limited. The serotonin 1A (5-HT1A) receptor has been implicated in the regulation of several brain functions, including mood, emotions, memory, and neuroplasticity, all of which are deleteriously affected by cerebral ischemia. This review focuses on the specific roles and mechanisms of 5-HT1A receptors in neuroprotection in experimental models of cerebral ischemia. We present experimental evidence that 5-HT1A receptor agonists can prevent neuronal damage and promote functional recovery induced by focal and transient global ischemia in rodents. However, indiscriminate activation of pre-and postsynaptic by non-biased 5-HT1A receptor agonists may be a limiting factor in the anti-ischemic clinical efficacy of these compounds since 5-HT1A receptors in different brain regions can mediate diverging or even contradictory responses. Current insights are presented into the 'biased' 5-HT1A post-synaptic heteroreceptor agonist NLX-101 (also known as F15599), a compound that preferentially and potently stimulates postsynaptic cortical pyramidal neurons without inhibiting firing of serotoninergic neurons, as a potential strategy providing neuroprotection in cerebral ischemic conditions.

5 HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors., Significance Statement-This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

Machado-Joseph disease (MJD) or Spinocerebellar ataxia type 3 (SCA3) is a progressive neurodegenerative disorder that affects movement coordination leading to a premature death. Despite several efforts, no disease-modifying treatment is yet available for this disease. Previous studies pinpointed the modulation of serotonergic signaling, through pharmacological inhibition of the serotonin transporter SERT, as a promising therapeutic approach for MJD/SCA3. Here, we describe the 5-HT

The emerging concept of "biased agonism" denotes the phenomenon whereby agonists can preferentially direct receptor signalling to specific intracellular responses among the different transduction pathways, thus potentially avoiding side effects and improving therapeutic effects. The aim of this study was to investigate biased agonism by using pharmacological magnetic resonance imaging (phMRI). The cerebral blood oxygen level dependent (BOLD) signal changes induced by increasing doses of two serotonin 5-HT1A receptor biased agonists, NLX-112 and NLX-101, were mapped in anaesthetized rats. Although both compounds display high affinity, selectivity and agonist efficacy for 5-HT1A receptors, NLX-101 is known to preferentially activate post-synaptic receptors, whereas NLX-112 targets both pre- and post-synaptic receptors. We used several doses of agonists in order to determine if the regional selectivity of NLX-101 was dose-dependent. NLX-112 and NLX-101 induced different positive and negative hemodynamic changes patterns at equal doses. Importantly, NLX-101 had no significant effect in regions expressing pre-synaptic receptors contrary to NLX-112. NLX-112 also produced higher BOLD changes than NLX-101 in the orbital cortex, the somatosensory cortex, and the magnocellular preoptic nuclei. In other regions such as the retrosplenial cortex and the dorsal thalamus, the drugs had similar effects. In terms of functional connectivity, NLX-112 induced more widespread changes than NLX-101. The present phMRI study demonstrates that two closely-related agonists display notable differences in their hemodynamic "fingerprints". These data support the concept of biased agonism at 5-HT1A receptors and raise the prospect of identifying novel therapeutics which exhibit improved targeting of brain regions implicated in neuropsychiatric disorders. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

The serotonin 5-HT1A receptor has attracted wide attention as a target for treatment of psychiatric disorders. Although this receptor is important in the pharmacological mechanisms of action of new-generation antipsychotics, its characterization remains incomplete. Studies based on in vitro molecular imaging on brain tissue by autoradiography, and more recently in vivo PET imaging, have not yielded clear results, in particular due to the limitations of current 5-HT1A radiotracers, which lack specificity and/or bind to all 5-HT1A receptors, regardless of their functional status. The new concept of PET neuroimaging of functionally active G-protein-coupled receptors makes it possible to revisit PET brain exploration by enabling new research paradigms. For the 5-HT1A receptor it is now possible to use [18F]-F13640, a 5-HT1A receptor radioligand with high efficacy agonist properties, to specifically visualize and quantify functionally active receptors, and to relate this information to subjects’ pathophysiological or pharmacological state. We therefore propose imaging protocols to follow changes in the pattern of functional 5-HT1A receptors in relation to mood deficits or cognitive processes. This could allow improved discrimination of different schizophrenia phenotypes and greater understanding of the basis of therapeutic responses to antipsychotic drugs. Finally, as well as targeting functionally active receptors to gain insights into the role of 5-HT1A receptors, the concept can also be extended to the study of other receptors involved in the pathophysiology or therapy of psychiatric disorders.

l-DOPA is the gold-standard pharmacotherapy for treatment of Parkinson's disease (PD) but can lead to the appearance of troubling dyskinesia which are attributable to ‘false neurotransmitter’ release of dopamine by serotonergic neurons. Reducing the activity of these neurons diminishes l-DOPA-induced dyskinesia (LID), but there are currently no clinically approved selective, high efficacy 5-HT1A receptor agonists. Here we describe the effects of NLX-112, a highly selective and efficacious 5-HT1A receptor agonist, on LID in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated marmosets, a non-human primate model of PD. NLX-112 exhibited modest plasma half-life (~2h) and marked plasma protein binding (96%). When administered to parkinsonian marmosets with l-DOPA (7 mg/kg p.o.), NLX-112 (0.025, 0.1 and 0.4 mg/kg p.o.) reduced LID scores at early time-points after administration, whilst only minimally interfering with the l-DOPA-induced reversal of motor disability. In contrast, the prototypical 5-HT1A receptor agonist, (+)8-OH-DPAT (0.6 and 2 mg/kg p. o.), reduced LID but also abolished l-DOPA's anti-disability activity. Administered by itself, NLX-112 (0.1, 0.2 mg/kg p.o.) produced very little dyskinesia or locomotor activity, but reduced motor disability scores by about half the extent elicited by l-DOPA, suggesting that it may have motor facilitation effects of its own. Both NLX-112 and (+)8-OH-DPAT induced unusual and dose-limiting behaviors in marmoset that resembled ‘serotonin behavioral syndrome’ observed previously in rat. Overall, the present study showed that NLX-112 has anti-LID activity at the doses tested as well as reducing motor disability. The data suggest that additional investigation of NLX-112 is desirable to explore its potential as a treatment for PD and PD-LID., Highlights • l-DOPA-induced dyskinesia (LID) develops in many Parkinson's disease patients. • NLX-112 is a highly selective and high efficacy serotonin 5-HT1A receptor agonist. • In MPTP-treated marmosets NLX112 reduced LID without impairing therapeutic activity. • NLX-112 by itself exhibited antiparkinsonian activity (decreased motor disability). • NLX-112 is a promising drug candidate for treatment of Parkinson's disease.

Biased agonism (or “functional selectivity”) at G-protein-coupled receptors has attracted rapidly increasing interest as a means to improve discovery of more efficacious and safer pharmacotherapeutics. However, most studies are limited to in vitro tests of cellular signaling and few biased agonists have progressed to in vivo testing. As concerns 5-HT1A receptors, which exert a major control of serotonergic signaling in diverse CNS regions, study of biased agonism has previously been limited by the poor target selectivity and/or partial agonism of classically available ligands. However, a new generation of highly selective, efficacious and druggable agonists has advanced the study of biased agonism at this receptor and created new therapeutic opportunities. These novel agonists show differential properties for G-protein signaling, cellular signaling (particularly pERK), electrophysiological effects, neurotransmitter release, neuroimaging by PET and pharmacoMRI, and behavioral tests of mood, motor activity and side effects. Overall, NLX-101 (a.k.a. F15599) exhibits preferential activation of cortical and brain stem 5-HT1A receptors, whereas NLX-112 (a.k.a. befiradol or F13640) shows prominent activation of 5-HT1A autoreceptors in Raphe nuclei and in regions associated with motor control. Accordingly, NLX-101 is potently active in rodent models of depression and respiratory control, whereas NLX-112 shows promising activity in models of Parkinson's disease across several species – rat, marmoset and macaque. Moreover, NLX-112 has also been labeled with 18F to produce the first agonist PET radiopharmaceutical (known as [18F]-F13640) for investigation of the active state of 5-HT1A receptors in rodent, primate and human. The structure-functional activity relationships of biased agonists have been investigated by receptor modeling and novel compounds have been identified which exhibit increased affinity at 5-HT1A receptors and new profiles of cellular signaling bias, notably for β-arrestin recruitment versus pERK. Taken together, the data suggest that 5-HT1A receptor biased agonists constitute potentially superior pharmacological agents for treatment of CNS disorders involving serotonergic mechanisms.

International audience; In neuropharmacology, the recent concept of 'biased agonism' denotes the capacity of certain agonists to target-specific intracellular pathways of a given receptor in specific brain areas. In the context of serotonin pharmacotherapy, 5-HT1A receptor-biased agonists could be of great interest in several neuropsychiatric disorders. The aim of this study was to determine whether biased agonists could be differentiated in terms of regional targeting by use of simultaneous functional magnetic resonance imaging (fMRI) and positron emission tomography (PET) brain imaging. We compared two 5-HT1A-biased agonists, NLX-112 and NLX-101, injected at three different doses in anaesthetised cats (n = 4). PET imaging was acquired for 90 min after bolus administration followed by constant infusion of the 5-HT1A radiotracer, [18F]MPPF. Drug occupancy was evaluated after injection at 50 min and BOLD fMRI was simultaneously acquired to evaluate subsequent brain activation patterns. 5-HT1A receptor occupancy was found to be dose-dependent for both agonists, but differed in magnitude and spatial distribution at equal doses with distinct BOLD patterns. Functional connectivity, as measured by BOLD signal temporal correlations between regions, was also differently modified by NLX-112 or NLX-101. Voxel-based correlation analyses between PET and fMRI suggested that NLX-112 stimulates both 5-HT1A autoreceptors and post-synaptic receptors, whereas NLX-101 preferentially stimulates post-synaptic cortical receptors. In cingulate cortex, the agonists induced opposite BOLD signal changes in response to receptor occupancy. These data constitute the first simultaneous exploration of 5-HT1A occupancy and its consequences in terms of brain activation, and demonstrates differential signalling by two 5-HT1A-biased agonists. Combined PET/fMRI represents a powerful tool in neuropharmacology, and opens new ways to address the concept of biased agonism by translational approaches.

Background and purpose: NLX-112 (a.k.a. befiradol, F13640) is a drug candidate intended for the treatment of l -DOPA-induced dyskinesia. It is a highly selective serotonin 5-HT1A receptor full agonist which has been previously tested in a variety of models of CNS effects including analgesic activity in rat. Its activity in mouse models of pain has not been previously investigated. Experimental approach The activity of NLX-112 was tested in mouse models of acute pain (hot plate), tonic pain (intraplantar formalin test), in the oxaliplatin-induced neuropathic pain model of chemotherapy-induced peripheral neuropathy and in the streptozotocin (STZ)-induced model of painful diabetic neuropathy. Key results The main findings indicate that (i) NLX-112 was markedly active in the formalin test with potent reduction of paw licking in both phases of the test (minimal effective dose (MED) 0.5 mg/kg i.p. and p.o. in acute phase, and 0.1 mg/kg i.p. and 1 mg/kg p.o. in late phase). The effects of NLX-112 in this test were completely abolished by the selective 5-HT1A receptor antagonist, WAY100635; (ii) NLX-112 was active in the hot plate test and in the oxaliplatin-induced neuropathic pain model of chemotherapy-induced peripheral neuropathy, but at markedly higher doses (MED 2.5 mg/kg i.p.); (iii) NLX-112 was least active in the STZ-induced model of painful diabetic neuropathy (MED 5 mg/kg i.p.); (iv) NLX-112 did not affect locomotor activity. Conclusions and implications NLX-112 may have significant potential for treatment of tonic pain but may be less promising as a candidate for treatment of chemotherapy-induced or diabetic neuropathic pain.

Objectives NLX-112 (befiradol, F13640) is a selective serotonin 5-HT1A receptor agonist. Although it has been tested in vivo, little has been reported on its in vitro signal transduction profile. Methods NLX-112 was tested on G-protein activation, inhibition of adenylyl cyclase, ERK1/2 phosphorylation (pERK) and receptor internalization in recombinant cell lines. NLX-112 was also tested on G-protein activation in rat hippocampal membranes. Gα subunit mRNA expression in cell lines and rat brain tissue was quantified by quantitative PCR. Key findings For all signalling measures, NLX-112 exhibited agonist efficacy greater than for reference compounds ((±)8-OH-DPAT or buspirone), but similar to the endogenous agonist, serotonin, and was more potent for pERK than other responses. In rat hippocampal membranes, NLX-112 stimulated ‘total G-proteins' but, unlike (±)8-OH-DPAT and buspirone, was more potent for Gαo activation. Cell lines predominantly expressed Gαi1 and Gαi2 mRNA, with low levels of Gαo, whereas in rat brain Gαo subunits showed highest mRNA expression. Conclusions Unlike reference compounds, NLX-112 was a highly efficacious agonist in vitro, preferentially activating pERK in cell lines and Gαo proteins in rat hippocampal membranes. However, Gα subunit mRNA levels differ markedly between rat brain and cell lines, warranting caution when extrapolating from recombinant systems to native tissues.

Despite extensive efforts to design serotonin 5-HT1A receptor compounds, there are currently no clinically available selective agonists to explore the therapeutic potential of activating this receptor. Commonly used drugs targeting 5-HT1A receptors, such as buspirone or other azapirone compounds, possess only limited selectivity over cross-reacting sites, act as partial agonists for 5-HT1A receptor activation, and are metabolically labile, generating active metabolites. In addition, drug discovery has been hampered by the multiplicity of 5-HT1A receptor subpopulations, expressed in different brain regions, that are coupled to distinct molecular signaling mechanisms and mediate a wide variety of physiological responses, both desired and undesired. : In this context, advances in 5-HT1A receptor drug discovery have attracted attention of novel ‘biased agonists’ that are selective, efficacious and preferentially target the brain regions that mediate therapeutic activity without triggering side effects. The prototypical first-in-class compound NLX-101 (a.k.a. F15599; 3-chloro-4-fluorophenyl-[4-fluoro-4-[[(5-methylpyrimidin-2-ylmethyl)amino]methyl]piperidin- 1-yl]methanone), preferentially activates 5-HT1A receptors in cortical regions and exhibits potent, rapidacting and sustained antidepressant-like and procognitive properties in animal models. : Here the background has been reviewed that led to the discovery of the class of 1-(1-benzoylpiperidin-4- yl)methanamine derivatives, including NLX-101, as well as recent advances in discovery of novel 5-HT1A receptor biased agonists, notably aryloxyethyl derivatives of 1‑(1-benzoylpiperidin-4yl)methanamine which show promising pharmacological activity both in vitro and in vivo. : Overall, the results suggest that opportunities exist for innovative drug discovery of selective 5-HT1A receptor biased agonists that may open new avenues for the treatment of CNS disorders involving dysfunction of serotonergic neurotransmission.

Novel 1-(1-benzoylpiperidin-4-yl)methanamine derivatives were designed as “biased agonists” of serotonin 5-HT1A receptors. The compounds were tested in signal transduction assays (ERK1/2 phosphorylation, cAMP inhibition, Ca2+ mobilization, and β-arrestin recruitment) which identified ERK1/2 phosphorylation-preferring aryloxyethyl derivatives. The novel series showed high 5-HT1A receptor affinity, >1000-fold selectivity versus noradrenergic α1, dopamine D2, serotonin 5-HT2A, histamine H1, and muscarinic M1 receptors, and favorable druglike properties (CNS-MPO, Fsp3, LELP). The lead structure, (3-chloro-4-fluorophenyl)(4-fluoro-4-(((2-(pyridin-2-yloxy)ethyl)amino)methyl)piperidin-1-yl)methanone (17, NLX-204), displayed high selectivity in the SafetyScreen44 panel (including hERG channel), high solubility, metabolic stability, and Caco-2 penetration and did not block CYP3A4, CYP2D6 isoenzymes, or P-glycoprotein. Preliminary in vivo studies confirmed its promising pharmacokinetic profile. 17 also robustly sti...

Item does not contain fulltext Brain serotonin (5-HT) plays a key role in aggressive behaviours and related psychopathologies, but its precise mechanism of action remains elusive. Genetic animal models may provide a tool to elucidate the relationship between aggression and serotonin. The present study showed that tryptophan hydroxylase 2 (Tph2) knockout (KO) rats, which exhibit profoundly diminished extracellular serotonin levels, display increased aggressiveness compared to their Tph2 wildtype (WT) counterparts. However, the level of aggression in Tph2 KO rats did not equal that of feral wild type Groningen (WTG) rats. To investigate whether enhanced 5-HT1A receptor functionality may be present in Tph2 KO rats, we tested the acute anti-aggressive potency of the highly selective 5-HT1A receptor full agonist NLX-112 (a.k.a. befiradol or F13640). Data show that compared to Tph2 WT and WTG rats, the NLX-112 dose-effect curve was shifted to the right in Tph2 KO animals. These results suggest that, unlike previous reports in Tph2 KO mice, Tph2 KO rats have a decreased 5-HT1A receptor sensitivity compared to both Tph2 WT and WTG animals.

Machado-Joseph disease (MJD) or Spinocerebellar ataxia type 3 (SCA3) is a progressive neurodegenerative disorder that affects movement coordination leading to a premature death. Despite several efforts, no disease-modifying treatment is yet available for this disease. Previous studies pinpointed the modulation of serotonergic signaling, through pharmacological inhibition of the serotonin transporter SERT, as a promising therapeutic approach for MJD/SCA3. Here, we describe the 5-HT1A receptor as a novel therapeutic target in MJD, using a C. elegans model of ATXN3 proteotoxicity. Chronic and acute administration of befiradol (also known as NLX-112), a highly specific 5-HT1A agonist, rescued motor function and suppressed mutant ATXN3 aggregation. This action required the 5-HT1A receptor orthologue in the nematode, SER-4. Tandospirone, a clinically tested 5-HT1A receptor partial agonist, showed a limited impact on animals' motor dysfunction on acute administration and a broader receptor activation profile upon chronic treatment, its effect depending on 5-HT1A but also on the 5-HT6/SER-5 and 5-HT7/SER-7 receptors. Our results support high potency and specificity of befiradol for activation of 5-HT1A/SER-4 receptors and highlight the contribution of the auto- and hetero-receptor function to the therapeutic outcome in this MJD model. Our study deepens the understanding of serotonergic signaling modulation in the suppression of ATXN3 proteotoxicity and suggests that a potent and selective 5-HT1A receptor agonist such as befiradol could constitute a promising therapeutic agent for MJD.

Pharmacological interventions that selectively activate serotonin 5-hydroxytryptramine-1A (5-HT1A) heteroreceptors may prevent or attenuate the consequences of brain ischemic episodes. The present study investigated whether the preferential 5-HT1A postsynaptic receptor agonist NLX-101 (a.k.a. F15599) mitigates cognitive and emotional impairments and affects neuroplasticity in mice that are subjected to the bilateral common carotid artery occlusion (BCCAO) model of brain ischemia. The selective serotonin reuptake inhibitor escitalopram (Esc) was used for comparative purposes because it is able to decrease morbidity and improve recovery in stroke patients and ischemic rodents. Sham and BCCAO mice received daily doses of NLX-101 (0.32 mg/kg, i.p) or Esc (20 mg/kg, i.p) for 28 days. During this period, they were evaluated for locomotor activity, anxiety- and despair-related behaviors and hippocampus-dependent cognitive function, using the open field, elevated zero maze, forced swim test and object location test, respectivelly. The mice's brains were processed for biochemical and histological analyses. BCCAO mice exhibited high anxiety and despair-like behaviors and performed worse than controls in the cognitive assessment. BCCAO induced neuronal and dendritic spine loss and decreases in the protein levels of neuronal plasticity markers, including brain-derived neurotrophic factor (BDNF), synaptophysin (SYN), and postsynaptic density protein-95 (PSD-95), in prefrontal cortex (PFC) and hippocampus. NLX-101 and Esc attenuated cognitive impairments and despair-like behaviors in BCCAO mice. Only Esc decreased anxiety-like behaviors due to brain ischemia. Both NLX-101 and Esc blocked the increase in plasma corticosterone levels and, restored BDNF, SYN and PSD-95 protein levels in the hippocampus. Moreover, both compounds impacted positively dentritic remodeling in the hippocampus and PFC of ischemic mice. In the PFC, NLX-101 increased the BDNF protein levels, while Esc in turn, attenuated the decrease in the PSD-95 protein levels induced by BCCAO. The present results suggest that activation of post-synaptic 5-HT1A receptors is the molecular mechanism for serotonergic protective effects in BCCAO. Moreover, post-synaptic biased agonists such as NLX-101 might constitute promising therapeutics for treatment of functional and neurodegenerative outcomes of brain ischemia.

Rationale Fear conditioning is an important factor in the etiology of anxiety disorders. Previous studies have demonstrated a role for serotonin (5-HT)1A receptors in fear conditioning. However, the relative contribution of somatodendritic 5-HT1A autoreceptors and post-synaptic 5-HT1A heteroreceptors in fear conditioning is still unclear. Objective To determine the role of pre- and post-synaptic 5-HT1A receptors in the acquisition and expression of cued and contextual conditioned fear. Methods We studied the acute effects of four 5-HT1A receptor ligands in the fear-potentiated startle test. Male Wistar rats were injected with the 5-HT1A receptors biased agonists F13714 (0–0.16 mg/kg, IP), which preferentially activates somatodendritic 5-HT1A autoreceptors, or F15599 (0–0.16 mg/kg, IP), which preferentially activates cortical post-synaptic 5-HT1A heteroreceptors, with the prototypical 5-HT1A receptor agonist R(+)8-OH-DPAT (0–0.3 mg/kg, SC) or the 5-HT1A receptor antagonist WAY100,635 (0–1.0 mg/kg, SC). Results F13714 (0.16 mg/kg) and R(+)-8-OH-DPAT (0.03 mg/kg) injected before training reduced cued fear acquisition. Pre-treatment with F15599 or WAY100,635 had no effect on fear learning. In the fear-potentiated startle test, F13714 (0.04–0.16 mg/kg) and R(+)-8-OH-DPAT (0.1–0.3 mg/kg) reduced the expression of cued and contextual fear, whereas F15599 had no effect. WAY100,635 (0.03–1.0 mg/kg) reduced the overall startle response. Conclusions The current findings indicate that activation of somatodendritic 5-HT1A autoreceptors reduces cued fear learning, whereas 5-HT1A receptors seem not involved in contextual fear learning. Moreover, activation of somatodendritic 5-HT1A autoreceptors may reduce cued and contextual fear expression, whereas we found no evidence for the involvement of cortical 5-HT1A heteroreceptors in the expression of conditioned fear.

Although serotonin 5-HT1A receptors constitute attractive therapeutic targets, there is a lack of potential clinical candidates that have a high degree of selectivity and full agonist efficacy. Recently, novel 5-HT1A receptor "biased agonists" F15599 (also known as NLX-101) and F13714 have been reported that exhibit distinctive properties for in vitro signaling, neurochemical, electrophysiological effects, and in brain imaging. The present study characterized their effects in rat models of anxiety (elevated plus-maze, EPM, and Vogel tests), in depressive-like behavior (forced swim test), and on the induction of the three serotonergic behaviors (forepaw treading, flat body posture, and lower lip retraction). The prototypical 5-HT1A receptor ligands (±)8-OH-DPAT and buspirone were tested as comparators. In the elevated plus-maze, F15599, F13714, and (±)8-OH-DPAT dose-dependently increased the amount and percentage of time spent in the open arms with minimal effective doses (MED) of 5 mg/kg p.o., 2.5 mg/kg p.o. and 1.25 mg/kg s.c., respectively. The effects of the three agonists were abolished by pretreatment with the selective 5-HT1A receptor antagonist, WAY100635 (0.63 mg/kg s.c.). Buspirone did not show significant activity in the EPM. In contrast, in the Vogel test only buspirone was active, significantly increasing the number of licks and shocks accepted (active dose: 1.25 mg/kg s.c.). However, WAY100635 failed to reverse the effects of buspirone in this test, suggesting that they were not 5-HT1A receptor-mediated. In the forced swim test, F15599, F13714, and (±)8-OH-DPAT were potently active, abolishing immobility (MED: 0.63 mg/kg p.o., 0.63 mg/kg p.o. and 0.16 mg/kg s.c., respectively). Buspirone was not active. In measures of serotonergic behavior, F13714 and (±)8-OH-DPAT robustly elicited all three signs of serotonergic behaviors, whereas F15599 and buspirone elicited only lower-lip retraction. Taken together, these observations highlight the distinct profiles of activity of 5-HT1A agonists and suggest that the novel biased agonist F15599 combines pronounced activity in a test of anxiety (elevated plus-maze) with potent antidepressant-like effects and low propensity to induce serotonergic behaviors. These data suggest that selective biased agonists could constitute promising pharmacotherapeutics for mood disorders.

Background and Purpose Pattern separation, that is, the formation of distinct representations from similar inputs, is an important hippocampal process implicated in cognitive domains like episodic memory. A deficit in pattern separation could lead to memory impairments in several psychiatric and neurological disorders. Hence, mechanisms by which pattern separation can be increased are of potential therapeutic interest. Experimental approach 5-HT1A receptors are involved in spatial memory. Herein we tested the ‘biased’ 5-HT1A receptor agonists F15599, which preferentially activates post-synaptic heteroreceptors, and F13714, which preferentially activates raphe-located autoreceptors, in rats in a novel spatial task assessing pattern separation, the object pattern separation (OPS) task. Key Results The acetylcholinesterase inhibitor donepezil, which served as a positive control, significantly improved spatial pattern separation at a dose of 1 mg·kg−1, p.o. F15599 increased pattern separation at 0.04 mg·kg−1, i.p., while F13714 decreased pattern separation at 0.0025 mg·kg−1, i.p. The selective 5-HT1A receptor antagonist WAY-100635 (0.63 mg·kg−1, s.c.) counteracted the effects of both agonists. These data suggest that acute preferential activation of post-synaptic 5-HT1A heteroreceptors improves spatial pattern separation, whereas acute preferential activation of raphe-located 5-HT1A autoreceptors impairs performance. Conclusions and Implications We successfully established and validated a novel, simple and robust OPS task and observed a diverging profile of response with ‘biased’ 5-HT1A receptor agonists based on their targeting of receptors in distinct brain regions. Our data suggest that the post-synaptic 5-HT1A receptor consists of a potential novel molecular target to improve pattern separation performance.

Summary Aims Positron emission tomography (PET) imaging using 5-HT1A receptor radioligands shows a decreased expression of this serotonin receptor in the hippocampus of patients with Alzheimer's disease (AD) at advanced stages. However, previous 5-HT1A receptor radioligands used in human imaging were antagonists, thought to bind to 5-HT1A receptors in different functional states (i.e., both the one which displays high affinity for agonists and is thought to mediate receptor activation, as well as the functional state which has low affinity for agonists). Comparing the PET imaging obtained using an agonist radioligand, which binds selectively to the functional state of the receptors, with the PET imaging obtained using an antagonist radioligand would therefore provide original information on 5-HT1A receptor impairment during AD. Methods Quantitative autoradiography using 18F-F15599 and 18F-MPPF, a 5-HT1A agonist and antagonist, respectively, was measured in hippocampi of 18 patients with AD. Results Functional 5-HT1A receptors, labeled by 18F-F15599, represented ~35% of total receptors, as estimated by 18F-MPPF labeling. 18F-F15599 binding decreased in dentate gyrus of patients with AD, as indicated by Braak's stages. In contrast, binding of 18F-MPPF was statistically unchanged. Conclusion These in vitro results support testing the concept of functional PET imaging using agonist radioligands in clinical studies.

Many dementia patients exhibit behavioral and psychological symptoms (BPSD), including psychosis and depression. Although antipsychotics are frequently prescribed off-label, they can have marked side effects. In addition, comparative preclinical studies of their effects are surprisingly scarce, and strategies for discovery of novel pharmacotherapeutics are lacking. We therefore compared eight antipsychotics in rat behavioral tests of psychosis, antidepressant-like activity, and cognitive impairment as a basis for preclinical evaluation of new drug candidates. The methods used in this study include inhibition of MK-801-induced hyperactivity, forced swim test (FST), passive avoidance (PA), spontaneous locomotor activity, and catalepsy. The drugs exhibited antipsychotic-like activity in the MK-801 test but with diverse profiles in the other models. Risperidone impaired PA performance, but with some dose separation versus its actions in the MK-801 test. In contrast, clozapine, olanzapine, lurasidone, and asenapine showed little or no dose separation in these tests. Aripiprazole did not impair PA performance but was poorly active in the MK-801 test. Diverse effects were also observed in the FST: chlorpromazine was inactive and most other drugs reduced immobility over narrow dose ranges, whereas clozapine reduced immobility over a wider dose range, overlapping with antipsychotic activity. Although the propensity of second-generation antipsychotics to produce catalepsy was lower, they all elicited pronounced sedation. Consistent with clinical data, most currently available second-generation antipsychotics induced cognitive and motor side effects with little separation from therapeutic-like doses. This study provides a uniform in vivo comparative basis on which to evaluate future early-stage drug candidates intended for potential pharmacotherapy of BPSD.

Background and Purpose Many dementia patients exhibit behavioural and psychological symptoms (BPSD) that include psychosis, aggressivity, depression and anxiety. Antipsychotic drugs are frequently prescribed but fail to significantly attenuate mood deficits, may interfere with cognitive function and are associated with motor and cardiac side effects, which are problematic in elderly patients. A need therefore exists for drugs that are better suited for the treatment of BPSD. Experimental Approach We used in vitro cellular and in vivo behavioural tests to characterize ADN-1184, a novel arylsulfonamide ligand with potential utility for treatment of BPSD. Key Results ADN-1184 exhibits substantial 5-HT6/5-HT7/5-HT2A/D2 receptor affinity and antagonist properties in vitro. In tests of antipsychotic-like activity, it reversed MK-801-induced hyperactivity and stereotypies and inhibited conditioned avoidance response (MED = 3 mg·kg−1 i.p.). Remarkably, ADN-1184 also reduced immobility time in the forced swim test at low doses (0.3 and 1 mg·kg−1 i.p.; higher doses were not significantly active). Notably, up to 30 mg·kg−1 ADN-1184 did not impair memory performance in the passive avoidance test or elicit significant catalepsy and only modestly inhibited spontaneous locomotor activity (MED = 30 mg·kg−1 i.p.). Conclusions and Implications ADN-1184 combines antipsychotic-like with antidepressant-like properties without interfering with memory function or locomotion. This profile is better than that of commonly used atypical antipsychotics tested under the same conditions and suggests that it is feasible to identify drugs that improve BPSD, without exacerbating cognitive deficit or movement impairment, which are of particular concern in patients with dementia.

International audience; Serotonin 1A (5-HT 1A) receptors are involved in several physiological and pathological processes and constitute therefore an important therapeutic target. The recent pharmacological concept of biased agonism asserts that highly selective agonists can preferentially direct receptor signaling to specific intracellular responses, opening the possibility of drugs targeting a receptor subtype in specific brain regions. The present study brings additional support to this concept thanks to functional magnetic resonance imaging (7 Tesla-fMRI) in anaesthetized rats. Three 5-HT 1A receptor agonists (8-OH-DPAT, F13714 and F15599) and one 5-HT 1A receptor antagonist (MPPF) were compared in terms of influence on the brain blood oxygen level-dependent (BOLD) signal. Our study revealed for the first time contrasting BOLD signal patterns of biased agonists in comparison to a classical agonist and a silent antagonist. By providing functional information on the influence of pharmacological activation of 5-HT 1A receptors in specific brain regions, this neuroimaging approach, translatable to the clinic, promises to be useful in exploring the new concept of biased agonism in neuropsychopharmacology. 5-HT 1A receptors belong to the family of serotonin receptors, composed of 13 receptor subtypes. 5-HT 1A receptors are known to play a key role in serotonin neurotransmission due to their localization both as pre-synaptic receptors located on serotonin cell bodies in the raphe nuclei (somatodendritic receptors) and as post-synaptic heteroreceptors in forebrain areas that receive serotonergic projections. In such areas, 5-HT 1A receptors are located on pyramidal and GABAergic neurons of the neocortex and limbic system 1,2. Because of their distribution pattern and of their central role in the modulation of the serotoninergic neurotransmission, 5-HT 1A receptors are involved in several physiological and pathological processes and constitute therefore an important therapeutic target for psychiatric 3 and, more recently, for neurological disorders 4. Indeed, it is now well establish that 5-HT 1A receptors are one of the main targets for the treatment of mood disorders 5 , with different actions that depend strongly on their localization. For example, activation of soma-todendritic receptors by serotonin or 5-HT 1A receptor agonists decreases the firing of serotonin neurons in the raphe, and, consequently decreases its terminal release 6. This decrease is thought to be partially responsible for the delay in onset of the therapeutic action of selective serotonin reuptake inhibitors (SSRI) antidepressants 5. A recent study renewed this concept, showing that expression levels of 5-HT 1A somatodendritic receptors, are critically important for SSRI treatment response by controlling serotoninergic tone 7. On the other hand, the activation of postsynaptic 5-HT 1A receptors seems to be equally important for response to antidepressants 5,8. In a different therapeutic area, 5-HT 1A receptor agonism is also known as an important feature of some atypical antipsychotics including clozapine, aripiprazole, ziprasidone and quetiapine 9–12. Recently, 5-HT 1A receptors have attracted renewed interest as possible targets in neuropharmacology. For example, it was described that blockade of post-synaptic 5-HT 1A receptors, located on pyramidal cells, can improve cognition by enhancing glutamatergic transmission 13. This led to clinical trials using the 5-HT 1A antagonist lecozotan as a procognitive drug in Alzheimer's disease 14,15. Other 5-HT 1A receptor ligands such as sarizotan, buspirone and tandospirone were shown to alleviate dyskinesia in Parkinson's disease patients 16,17. More recently

Background In vivo mapping by positron emission tomography of the serotonin 1A receptors has been hindered by the lack of suitable agonist positron emission tomography probes. 18 F-labeled F13714 is a recently developed biased agonist positron emission tomography probe that preferentially targets subpopulations of serotonin 1A receptors in their " active state, " but its brain labeling pattern in nonhuman primate has not been described. In addition, a potential confound in the translatability of PET data between nonhuman animal and human arise from the use of anesthetics that may modify the binding profiles of target receptors. Methods Positron emission tomography scans were conducted in a cohort of common marmosets (n = 4) using the serotonin 1A receptor biased agonist radiotracer, 18 F-F13714, compared with a well-characterized 18 F-labeled antagonist radiotracer, 18 F-MPPF. Experiments on each animal were performed under both consciousness and isoflurane-anesthesia conditions. Results 18 F-F13714 binding distribution in marmosets by positron emission tomography differs markedly from that of the of the 18F-MPPF. Whereas 18F-MPPF showed highest binding in hippocampus and amygdala, 18F-F13714 showed highest labeling in other regions, including insular and cingulate cortex, thalamus, raphe, caudate nucleus, and putamen. The binding potential values of 18F-F13714 were about one-third of those observed with 18F-MPPF, with marked individual- and region-specific differences under isoflurane-anesthetized vs conscious conditions. Conclusions These findings highlight the importance of investigating the brain imaging of serotonin 1A receptors using agonist probes such as 18F-F13714, which may preferentially target subpopulations of serotonin 1A receptors in specific brain regions of nonhuman primate as a biased agonist.

Background and purpose: F15599, a novel 5-hydroxytryptamine (5-HT)1A receptor agonist with 1000-fold selectivity for 5-HT compared with other monoamine receptors, shows antidepressant and procognitive activity at very low doses in animal models. We examined the in vivo activity of F15599 at somatodendritic autoreceptors and postsynaptic 5-HT1A heteroreceptors. Experimental approach: In vivo single unit and local field potential recordings and microdialysis in the rat. Key results: F15599 increased the discharge rate of pyramidal neurones in medial prefrontal cortex (mPFC) from 0.2 µg·kg−1 i.v and reduced that of dorsal raphe 5-hydroxytryptaminergic neurones at doses >10-fold higher (minimal effective dose 8.2 µg·kg−1 i.v.). Both effects were reversed by the 5-HT1A antagonist (±)WAY100635. F15599 did not alter low frequency oscillations (∼1 Hz) in mPFC. In microdialysis studies, F15599 increased dopamine output in mPFC (an effect dependent on the activation of postsynaptic 5-HT1A receptors) with an ED50 of 30 µg·kg−1 i.p., whereas it reduced hippocampal 5-HT release (an effect dependent exclusively on 5-HT1A autoreceptor activation) with an ED50 of 240 µg·kg−1 i.p. Likewise, application of F15599 by reverse dialysis in mPFC increased dopamine output in a concentration-dependent manner. All neurochemical responses to F15599 were prevented by administration of (±)WAY100635. Conclusions and implications: These results indicate that systemic administration of F15599 preferentially activates postsynaptic 5-HT1A receptors in PFC rather than somatodendritic 5-HT1A autoreceptors. This regional selectivity distinguishes F15599 from previously developed 5-HT1A receptor agonists, which preferentially activate somatodendritic 5-HT1A autoreceptors, suggesting that F15599 may be particularly useful in the treatment of depression and of cognitive deficits in schizophrenia.

Background and purpose: Activation of post-synaptic 5-HT1A receptors may provide enhanced therapy against depression. We describe the signal transduction profile of F15599, a novel 5-HT1A receptor agonist. Experimental approach: F15599 was compared with a chemical congener, F13714, and with (+)8-OH-DPAT in models of signal transduction in vitro and ex vivo. Key results: F15599 was highly selective for 5-HT1A receptors in binding experiments and in [35S]-GTPγS autoradiography of rat brain, where F15599 increased labelling in regions expressing 5-HT1A receptors. In cell lines expressing h5-HT1A receptors, F15599 more potently stimulated extracellular signal-regulated kinase (ERK1/2) phosphorylation, compared with G-protein activation, internalization of h5-HT1A receptors or inhibition of cAMP accumulation. F13714, (+)8-OH-DPAT and 5-HT displayed a different rank order of potency for these responses. F15599 stimulated [35S]-GTPγS binding more potently in frontal cortex than raphe. F15599, unlike 5-HT, more potently and efficaciously stimulated Gαi than Gαo activation. In rat prefrontal cortex (a region expressing post-synaptic 5-HT1A receptors), F15599 potently activated ERK1/2 phosphorylation and strongly induced c-fos mRNA expression. In contrast, in raphe regions (expressing pre-synaptic 5-HT1A receptors) F15599 only weakly or did not induce c-fos mRNA expression. Finally, despite its more modest affinity in vitro, F15599 bound to 5-HT1A receptors in vivo almost as potently as F13714. Conclusions and implications: F15599 showed a distinctive activation profiles for 5-HT1A receptor-mediated signalling pathways, unlike those of reference agonists and consistent with functional selectivity at 5-HT1A receptors. In rat, F15599 potently activated signalling in prefrontal cortex, a feature likely to underlie its beneficial effects in models of depression and cognition.

Background and purpose: The D2/D3 receptor antagonist, D4 receptor partial agonist, and high efficacy 5-HT1A receptor agonist F15063 was shown to be highly efficacious and potent in rodent models of activity against positive symptoms of schizophrenia. However F15063 induced neither catalepsy nor the ‘serotonin syndrome’. Here, we evaluated its profile in rat models predictive of efficacy against negative symptoms/cognitive deficits of schizophrenia. Experimental approach: F15063, given i.p., was assessed in models of behavioural deficits induced by interference with the NMDA/glutamatergic (phencyclidine: PCP) or cholinergic (scopolamine) systems. Key results: Through 5-HT1A activation, F15063 partially alleviated (MED: 0.04 mg kg−1) PCP-induced social interaction deficit between two adult rats, without effect by itself, underlining its potential to combat negative symptoms. At doses above 0.16 mg kg−1, F15063 reduced interaction by itself. F15063 (0.16 mg kg−1) selectively re-established PCP-impaired ‘cognitive flexibility’ in a reversal learning task, suggesting potential against adaptability deficits. F15063 (0.04–0.63 mg kg−1) also reversed scopolamine-induced amnesia in a juvenile-adult rat social recognition test, indicative of a pro-cholinergic influence. Activity in this latter test is consistent with its D4 partial agonism, as it was blocked by the D4 antagonist L745,870. Finally, F15063 up to 40 mg kg−1 did not disrupt basal prepulse inhibition of startle reflex in rats, a marker of sensorimotor gating. Conclusions and implications: The balance of D2/D3, D4 and 5-HT1A receptor interactions of F15063 yields a promising profile of activity in models of cognitive deficits and negative symptoms of schizophrenia. British Journal of Pharmacology (2007) 151, 266–277. doi:10.1038/sj.bjp.0707160

Background and purpose: F15063 is a high affinity D2/D3 antagonist, D4 partial agonist, and high efficacy 5-HT1A agonist, with little affinity (40-fold lower than for D2 receptors) at other central targets. Here, the profile of F15063 was evaluated in models of positive symptoms of schizophrenia and motor side-effects. Experimental approach: Rodent behavioural tests were based on reversal of hyperactivity induced by psychostimulants and on measures of induction of catalepsy and ‘serotonin syndrome’. Key results: F15063 potently (ED50s: 0.23 to 1.10 mg kg−1 i.p.) reversed methylphenidate-induced stereotyped behaviors, blocked d-amphetamine and ketamine hyperlocomotion, attenuated apomorphine-induced prepulse inhibition (PPI) deficits, and was active in the conditioned avoidance test. In mice, it reversed apomorphine-induced climbing (ED50 = 0.30 mg kg−1 i.p.). F15063, owing to its 5-HT1A agonism, did not produce (ED50 > 40 mg kg−1 i.p.) catalepsy in rats and mice, a behavior predictive of occurrence of extra-pyramidal syndrome (EPS) in man. This absence of cataleptogenic activity was maintained upon sub-chronic treatment of rats for 5 days at 40 mg kg−1 p.o. Furthermore, F15063 did not induce the ‘serotonin syndrome’ in rats (flat body posture and forepaw treading: ED50 >32 mg kg−1 i.p.). Conclusions and implications: F15063 conformed to the profile of an atypical antipsychotic, with potent actions in models of hyperdopaminergic activity but without inducing catalepsy. These data suggest that F15063 may display potent antipsychotic actions with low EPS liability. This profile is complemented by a favourable profile in rodent models of negative symptoms and cognitive deficits of schizophrenia (companion paper). British Journal of Pharmacology (2007) 151, 253–265. doi:10.1038/sj.bjp.0707159

Background and purpose: Combining 5-HT1A receptor activation with dopamine D2/D3 receptor blockade should improve negative symptoms and cognitive deficits in schizophrenia. We describe the in vitro profile of F15063 (N-[(2,2-dimethyl-2,3-dihydro-benzofuran-7-yloxy)ethyl]-3-(cyclopent-1-enyl)-benzylamine). Experimental approach: F15063 was characterised in tests of binding affinity and in cellular models of signal transduction at monoamine receptors. Key results: Affinities (receptor and pKi values) of F15063 were: rD2 9.38; hD2L 9.44; hD2S 9.25; hD3 8.95; hD4 8.81; h5-HT1A 8.37. F15063 had little affinity (40-fold lower than D2) at other targets. F15063 antagonised dopamine-activated G-protein activation at hD2, rD2 and hD3 receptors with potency (pKb values 9.19, 8.29 and 8.74 in [35S]GTPγS binding experiments) similar to haloperidol. F15063 did not exhibit any hD2 receptor agonism, even in tests of ERK1/2 phosphorylation and G-protein activation in cells with high receptor expression. In contrast, like (±)8-OH-DPAT, F15063 efficaciously activated h5-HT1A (Emax 70%, pEC50 7.57) and r5-HT1A receptors (52%, 7.95) in tests of [35S]GTPγS binding, cAMP accumulation (90%, 7.12) and ERK1/2 phosphorylation (93%, 7.13). F15063 acted as a partial agonist for [35S]GTPγS binding at hD4 (29%, 8.15) and h5-HT1D receptors (35%, 7.68). In [35S]GTPγS autoradiography, F15063 activated G-proteins in hippocampus, cortex and septum (regions enriched in 5-HT1A receptors), but antagonised quinelorane-induced activation of D2/D3 receptors in striatum. Conclusions and implications: F15063 antagonised dopamine D2/D3 receptors, a property underlying its antipsychotic-like activity, whereas activation of 5-HT1A and D4 receptors mediated its actions in models of negative symptoms and cognitive deficits of schizophrenia (see companion papers). British Journal of Pharmacology (2007) 151, 237–252. doi:10.1038/sj.bjp.0707158

Background The serotonin (5-HT) deficiency hypothesis of aggression is being seriously challenged by pharmacological data showing robust anti-aggressive effects of 5-HT1A receptor agonists in dose ranges that concomitantly inhibit 5-HT neurotransmission. Hence, an adequate interpretation of the role of 5-HT activity in regulating aggression depends on elucidating the predominant site of action, i.e., raphe presynaptic autoreceptors versus forebrain postsynaptic heteroreceptors, of these 5-HT1A receptor agonists. Objectives The present experiments investigated the anti-aggressive properties of the selective 5-HT1A receptor agonists F15599 that preferentially target postsynaptic 5-HT1A heteroreceptors in the frontal cortex and F13714 that more preferentially activates raphe somatodendritic 5-HT1A autoreceptors. Methods Both ‘biased’ agonists were acutely administered intraperitoneally in aggressive resident male WTG rats confronting an intruder. Results Systemic administration of F15599 and F13714 exerted very potent (ID50 = 0.095 and 0.0059 mg/kg, respectively) anti-aggressive effects. At 4.5-fold higher dose ranges, the anti-aggressive effects were accompanied by concomitant motor inactivity and/or reduction of social engagement. Pretreatment with WAY-100635 counteracted the behavioural effects of both agonists. Conclusions Overall, the qualitatively similar but quantitatively different anti-aggressive profiles of F15599 and F13714 largely correspond to their distinct 5-HT1A receptor binding/activation potencies. Moreover, the marked anti-aggressive potency of F13714 adds additional support for a critical role of raphe somatodendritic 5-HT1A autoreceptors, and hence phasic 5-HT neuron activity, in the initiation/execution of aggressive actions.

Background and purpose: Desensitization of somatodendritic 5-HT1A receptors is involved in the mechanism of action of several antidepressants, but the rapidity of this effect and the amount of agonist stimulation needed are unclear. We evaluated the capacity of the high-efficacy 5-HT1A agonist, F13714 (3-chloro-4-fluorophenyl-(4-fluoro-4-{[(5-methyl-6-methylaminopyridin-2-ylmethyl)-amino]-methyl}-piperidin-1-yl-methanone) and of the partial agonist, flesinoxan, to desensitize somatodendritic 5-HT1A receptors involved in the control of 5-HT release. Experimental approach: Intracerebral microdialysis in the hippocampus of freely moving rats was used to examine the acute and chronic effects of the two compounds (administered by osmotic pumps for 3, 7 or 14 days) on extracellular 5-HT levels, measured by HPLC with electrochemical detection. Key results: When given acutely, F13714, flesinoxan and the low-efficacy 5-HT1A agonist, buspirone, dose-dependently decreased extracellular 5-HT concentrations (ED50 values: 0.04, 0.77 and 5.6 mg kg � 1 , respectively). The selective 5-HT1A antagonist WAY100635 inhibited the effects of the three compounds. F13714 (2.5 mg kg � 1 per day for 3, 7 or 14 days and 0.63 mg kg � 1 for 7 days) significantly attenuated the inhibition of 5-HT release induced by buspirone (10 mg kg � 1 ). In contrast, flesinoxan (10 mg kg � 1 per day) failed to alter the response to buspirone at any of the treatment durations. Conclusions and implications: Rat somatodendritic 5-HT1A receptors controlling hippocampal 5-HT release were rapidly desensitized by chronic activation with a high-efficacy 5-HT1A agonist, but not by chronic activation with a partial agonist. Thus, rapid 5-HT1A autoreceptor desensitization by high-efficacy agonists may accelerate the onset of the therapeutic effects of antidepressants.

The dopamine D1/D2 agonist apomorphine (0.63 mg/kg) disrupted prepulse inhibition (PPI) of acoustic startle in rats, a model of sensorimotor gating deficits observed in schizophrenia. All current antipsychotics, which antagonize D2 receptors, prevent this apomorphine-induced deficit. A novel class of antipsychotics possesses, in addition to D2 antagonist property, various levels of 5-HT1A agonist activity. Considering that the latter itself produces PPI deficits, it appeared necessary to assess the potential of this novel class of antipsychotics to reverse apomorphine-PPI deficits. Potent D2 antagonists, like haloperidol (0.63-2.5 mg/kg), risperidone (0.63-10 mg/kg), and olanzapine (0.63-40 mg/kg) prevented apomorphine PPI disruption. The atypical antipsychotics, clozapine (40 mg/kg), nemonapride (0.01-2.5 mg/kg), ziprasidone (10 mg/kg), and aripiprazole (0.01 and 10 mg/kg), which all exhibit 5-HT1A agonist properties, reversed PPI deficits at some doses only, whereas the anti-dyskinetic agent sarizotan (0.16-10 mg/kg), an efficacious 5-HT1A agonist, did not. New generation antipsychotics with marked 5-HT1A agonist properties, such as SLV313 and SSR181507 (0.0025-10 mg/kg and 0.16-10 mg/kg, respectively) did not reverse these deficits whereas bifeprunox (0.04-2.5 mg/kg) did. To reveal the contribution of 5-HT1A agonist properties in the lack of effects of SLV313 and SSR181507, we pretreated rats with the 5-HT1A antagonist WAY100635 (0.63 mg/kg). Under these conditions, significant reversal of PPI deficit was observed, indicating that D2 antagonist properties of SLV313 and SSR181507 are now sufficient to overcome the disruptive effects of apomorphine. To summarize, antipsychotics possessing agonist efficacy at 5-HT1A receptors exhibit diverse profiles against apomorphine-induced PPI deficits, depending on the balance between D2 and 5-HT1A activities, suggesting that they may display distinct activity on some aspects of gating deficits in schizophrenic patients.

Serotonin 5-HT1B receptors couple to G-proteins of the Gi/o family. However, their activation of specific G-protein subtypes is poorly characterised. Using an innovative antibody capture/guanosine-5′-0-(3-[35S]thio)-triphosphate ([35S]GTPγS) binding strategy, we characterised Gαi3 subunit activation by h5-HT1B receptors stably expressed in Chinese hamster ovary (CHO) cells. The agonists, 5-HT, alniditan and BMS181,101, stimulated Gαi3, whereas methiothepin and SB224,289 behaved as inverse agonists. The selective 5-HT1B receptor ligand, S18127, modestly stimulated Gαi3 and reversed the actions of both 5-HT and methiothepin. S18127 (1 μM) also produced parallel, dextral shifts of the 5-HT and methiothepin isotherms. Isotopic dilution experiments ([35S]GTPγS versus GTPγS) revealed high-affinity [35S]GTPγS binding to Gαi3 subunits in the absence of receptor ligands indicating constitutive activity. High-affinity [35S]GTPγS binding was increased 2.8-fold by 5-HT with an increase in the affinity of GTPγS for Gαi3 subunits. In contrast, methiothepin halved the number of high-affinity binding sites and decreased their affinity. h5-HT1B receptor-mediated Gαi3 subunit activation was dependent on the concentration of NaCl. At 300 mM, 5-HT stimulated [35S]GTPγS binding, basal Gαi3 activation was low and methiothepin was inactive. In contrast, at 10 mM NaCl, basal activity was enhanced and the inverse agonist activity of methiothepin was accentuated. Under these conditions, 5-HT decreased Gαi3 activation. In conclusion, at h5-HT1B receptors expressed in CHO cells: (i) inverse agonist induced inhibition of Gαi3, and its reversal by S18127, reveals constitutive activation of this Gα subunit; (ii) constitutive Gαi3 activation can be quantified by isotopic dilution [35S]GTPγS binding and (iii) decreasing NaCl concentrations enhances Gαi3 activation and leads to protean agonist properties of 5-HT: that is a switch to inhibition of Gαi3. British Journal of Pharmacology (2003) 138, 1077–1084. doi:10.1038/sj.bjp.0705140